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Webmin is a web-based interface for system administration for Unix. Using any modern web browser, you can setup user accounts, Apache, DNS, file sharing and much more. Webmin removes the need to manually edit Unix configuration files like /etc/passwd, and lets you manage a system from the console or remotely.

Ubuntu in particular don't allow logins by the root user by default. However, the user created at system installation time can use sudo to switch to root. Webmin will allow any user who has this sudo capability to login with full root privileges.

I would’t say “must”. Just because you enable ssh server, webmin or even apache doesn’t mean they are public facing. Almost all servers are going to be sitting behind a router/firewall with a private ip. You typically have to choose what to forward. And if you are capable of that configuration then hopefully you are aware of the risks.